1. Field of the Invention
The present invention generally relates to a multi-type air conditioner system and, more particularly, to a multi-type air conditioner system comprising a single outdoor unit and a plurality of indoor units.
2. Description of the Related Art
As is well known, in a multi type air conditioner system comprising a single outdoor unit and a plurality of indoor units, the outdoor unit has a compressor, a four-way valve, and an outdoor heat exchanger, and each indoor unit has an indoor heat exchanger. Parallel circuits of the compressor, the four-way valve, the outdoor heat exchanger, and the respective indoor heat exchangers are connected to constitute a plurality of heat pump type refrigeration cycles. An electronic expansion valve is provided to a liquid pipe between the outdoor heat exchanger and each indoor heat exchanger, and an electronic flow adjustment valve is provided to a gas pipe between each indoor heat exchanger and the four-way valve.
That is, in the cooling operation mode, the refrigerant discharged from the compressor is flowed through the four-way valve, the outdoor heat exchanger, the respective electronic expansion valves, the respective indoor heat exchangers, and the respective flow adjustment valves to cause the outdoor heat exchanger and the respective indoor heat exchangers to serve as a condenser and evaporators, respectively. The opening degrees of the respective flow adjustment valves are controlled in accordance with the required capacities of the respective indoor units, and the refrigerant super-heat degrees in the respective indoor heat exchangers serving as the evaporators are detected. The opening degrees of the respective electronic expansion valves are controlled so that the detected refrigerant super-heat degrees become preset values.
In the heating operation mode, the refrigerant discharged from the compressor is flowed through the respective flow adjustment valves, the respective indoor heat exchangers, the respective electronic expansion valves, and the outdoor heat exchanger to cause the respective indoor heat exchangers and the outdoor indoor heat exchanger to serve as condensers and an evaporator, respectively. The opening degrees of the respective flow adjustment valves are controlled in accordance with the required capacities of the respective indoor units, and the refrigerant super-heat degree in the outdoor heat exchanger serving as the evaporator is detected. The opening degrees of the respective electronic expansion valves are controlled so that the detected refrigerant super-heat degree becomes a preset value.
Upon starting of the operation, i.e., upon starting of the compressor, first, the liquid expansion valves are opened to predetermined opening degrees and this state is kept for a predetermined period of time. Then, normal opening degree control in accordance with the refrigerant super-heat degree is started.
In this case, the predetermined opening degrees are kept first. This is to cope with the unstable refrigerant super-heat degree itself immediately after start of the operation.
Regarding a stopped (including idling) indoor unit, the corresponding electronic expansion valve and flow adjustment valve are completely opened to block in-flow of the refrigerant.
In the above conventional multi-type air conditioner system, since optimum control is not necessarily performed for the gaseous flow adjustment valves and the liquid expansion valves, many problems arise as follows.
I. First, as a control scheme of a refrigerant super-heat degree of an air conditioner system, PID control is generally performed.
In a multi-type air conditioner system, however, a change in operating state of each indoor unit appears as a large state change in the refrigeration cycle as a control target. For this reason, the PID control, which determines control parameters of the gaseous flow adjustment valves and the liquid expansion valves by regarding the control target as a linear system, lacks flexibility against a change in state of the refrigeration cycle. A refrigerant super-heat degree may overshoot the preset value, or a hunting may occur.
II. In the conventional multi-type air conditioner system, upon start of the compressor, the liquid electronic expansion valves are first kept at predetermined opening degrees. However, this causes an undesirable state in which the suction-side refrigerant pressure of the compressor is largely decreased.
A refrigerant suction amount of a compressor is determined by a difference in refrigerant pressure between the discharge side and the suction side. Therefore, when the suction-side refrigerant pressure is largely decreased, as described above, the refrigerant suction amount of the compressor is increased, and the liquid refrigerant stored in the low-pressure-side pipe of the refrigeration cycle is quickly drawn by vacuum in the compressor. This so-called liquid return phenomenon is not preferable as it damages the compressor.
III. The liquid electronic expansion valve and the gaseous flow adjustment valve corresponding to a stopped (including idling) indoor unit are completely opened.
Then, the refrigerant is stored in the indoor heat exchanger, and shortage of the refrigerant flow amount in the refrigeration cycle as a whole may occur. In other words, the capability may become short in an operating indoor unit.
In order to solve this problem, a refrigerant return bypass having a capillary tube may be provided to recover the refrigerant. Bypasses are, however, required corresponding in number to the indoor units, complicating the configuration and increasing the cost.
IV. In the heating operation mode, a large pressure acts on two sides of a liquid flow adjustment valve corresponding to a stopped (including idling) indoor unit.
As a result, when the liquid flow adjustment valve is opened upon start (including resumption) of the operation under a state in which the pressure is not balanced, loud refrigerant noise may occur to annoy people around the indoor unit. In addition, the liquid flow adjustment valve needs to have a large drive motor in order to cope with the large pressure difference, resulting in an increase in cost.
In order to solve this problem, the capability of the compressor may be decreased before opening the liquid flow adjustment valve. Then, however, the compressor cannot cope with the requested capabilities of the respective indoor units. Indoor temperature control may be adversely affected to impair the amenity.